1. Field of the Invention
This invention relates to a focus detecting device of a microscope, and more particularly to an automatic focusing device of a phase-contrast microscope.
2. Description of the Prior Art
Generally, an automatic focusing device for an optical instrument such as a microscope or the like photoelectrically converts the image of an object by an objective lens into an image signal, extracts the high frequency component of this image signal to extract the high frequency component of the space frequency component of the image, and adjusts the relative distance between the object and the objective lens so that this high frequency component becomes maximum. Such an automatic focusing device is simple, but it has been found that simple detection of the maximum of the high frequency component of the image signal may sometimes lead to wrong focus detection. For example, in the automatic focusing device used when a specimen for testing HLA (human leukocyte antigen), particularly for carrying out microdeoplet lymphocyte cytotoxicity lest is observed through a phase-contrast microscope, the value of the high frequency component of the image signal when the relative distance between the object, i.e. said specimen, and the objective lens of the microscope has been varied has been found to be as follows:
(1) As shown in FIG. 1 of the accompanying drawings, a maximum peak P.sub.1 of the high frequency component lies at a point l.sub.1 slightly spaced apart from the in-focus position BF by a predetermind distance lc, and at points l.sub.2, l.sub.3 at the opposite sides of this maximum peak P.sub.1 spaced apart therefrom by a substantially predetermined equidistance, second and third peaks P.sub.2 and P.sub.3 considerably smaller than the maximum peak value P.sub.1 but substantially equal in value to each other appear. Here it is to be understood that P.sub.2 &gt;P.sub.3.
(2) In a phase-contrast objective lens, it is sometimes the case that depending on the state of the cells of the specimen, particularly the positional relation between the cells, or the presence of flaws of the container containing the specimen therein, as shown in FIG. 2 of the accompanying drawings, a great peak P.sub.0 comparable to the maximum peak P.sub.1 attributable to the in-focus position BF occurs at a position l.sub.0 greatly spaced from the in-focus position BF.
(3) Where bubbles or dust exists in the sample, a number of peaks substantially equal in height appear as shown in FIG. 3 of the accompanying drawings.
The above experimental facts show that for the image of the phase-contrast optical system, simply regarding the position providing the maximum value of the high frequency component as the in-focus position may lead to the danger of regarding an out-of-focus position as the in-focus position.